The present invention relates generally to wireless communication systems and more particularly to techniques for estimating mobile station location in a wireless code division multiple access (CDMA) system.
Demand for wireless communication services, such as mobile telephones in cellular and Personal Communications Service (PCS) systems, is continually increasing. An important issue in wireless communication systems involves the estimation of mobile station location. For example, the Federal Communications Commission (FCC) has requested that all cellular and PCS systems eventually include emergency 911 caller location capabilities similar to those provided in wired systems. As reported in Radio Communications Report, Vol. 15, No. 51, Dec. 16, 1996, the FCC has required that Phase I of a wireless emergency 911 (E-911) system providing a 911 agent with caller number and cell site location must be completed by Apr. 1, 1998, while Phase II of the E-911 system providing caller latitude and longitude within a radius of no more than 125 meters in at least 67% of all cases must be completed by Oct. 1, 2001. A number of other services requiring mobile location estimation are also being considered, including routing guidance services, fleet management and local commercial services. A wireless system which is able to determine the position of a given mobile station in an efficient manner could thus provide an enhanced level of service to the user, while meeting the above-noted FCC requirements and also generating additional revenue for the service provider. It would also be very advantageous if mobile location estimation could be incorporated into a given system without the need for any significant change to the standards on which the system is based.
In order to estimate mobile location with an acceptable degree of accuracy, either the mobile station needs to be able to detect signals from at least three surrounding base stations, or at least three surrounding base stations need to be able to detect a signal from the mobile station. The resulting signal propagation delay information can then be processed in a conventional manner using triangular relationships to derive an estimate of mobile location. Two important aspects of mobile location estimation thus involve the manner in which the signals are detected in the mobile station or base stations, and the accuracy of the propagation delay measurements required between the mobile station and base stations. An exemplary prior art mobile estimation technique is described in M. Wylie et al., xe2x80x9cThe Non-Line of Sight Problem in Mobile Location Estimation,xe2x80x9d ICUPC ""95, Boston, Mass. 1995, which is incorporated by reference herein.
In the case of a code division multiple access (CDMA) systems such as those based on the IS-95 standard, the implementation of a mobile location estimation capability presents a number of problems. Although CDMA systems spread signals over a wider frequency spectrum than narrowband systems such as TDMA, GSM and analog FM, and are therefore better able to resolve path delay ambiguity, the detection of signals from surrounding base stations by the mobile or detection of mobile signals by the surrounding base stations presents a greater challenge in CDMA systems than in narrowband systems. A basic principle of CDMA systems involves the use of power control to solve near-field problems and to control interference such that a capacity advantage can be achieved. Therefore, when the mobile station is not in a handoff zone, that is, when the mobile station is close to a base station, the signal strength from the surrounding base stations is very weak. Similarly, the power transmitted from the mobile is purposely made very small in order to prevent interference. This means that in order to achieve the capability of detecting signals from surrounding base stations in the mobile station or detecting a signal from the mobile station in surrounding base stations, an excessively large signal-to-noise gain may be required at the corresponding receivers. Application of conventional mobile location techniques to CDMA systems may therefore require alteration of basic system parameters, thereby increasing the cost and complexity of the system and possibly degrading system performance in terms of interference. An exemplary CDMA mobile estimation technique is described in J. Caffery et al., xe2x80x9cRadio Location in Urban CDMA Microcells,xe2x80x9d Proceedings of PIMRC ""95, pp. 858-862, IEEE, 1995, which is incorporated by reference herein. There are a number of problems with this prior art technique and other similar techniques. For example, such techniques typically utilize either coarse timing acquisition or a finer-acquisition delay lock loop to obtain path delay information. As noted above, it is difficult to detect a mobile station signal at the surrounding base stations unless an excessively large signal-to-noise gain is obtained, and the coarse timing and delay lock loop techniques have failed to solve this problem. In addition, the use of a delay lock loop usually requires the surrounding base stations to detect the mobile station signal either continuously or for a very long detection period, thereby wasting system resources and significantly increasing system complexity. Moreover, it is generally not feasible to utilize a delay lock loop in conjunction with an increased mobile station power level to assist acquisition because the resulting interference would substantially reduce system capacity.
As is apparent from the above, a need exists for an improved technique for estimating mobile station location in a wireless communication system, and which can be implemented in a CDMA system without requiring any significant alteration to system operating and performance standards.
The present invention provides methods and apparatus for determining mobile location in a wireless communication system. The invention allows a mobile station to transmit a location signal which can be detected and processed in a primary base station and a number of surrounding base stations in order to generate accurate path delay measurements. The primary base station controls characteristics of the location signal such as power level and transmission duration in order to facilitate detection by surrounding base stations while minimizing the interference created by the location signal within the system.
In accordance with one aspect of the present invention, a mobile location estimation technique suitable for use at initiation of a call or page response is provided. A mobile station transmits an access request signal and waits for an acknowledgment from a base station. A given base station receiving the access request signal is designated as a primary base station and has a primary location processor associated therewith. The primary location processor notifies a number of surrounding base stations, typically two other base stations, to prepare for detection of a location signal from the mobile station. The primary base station then transmits an access acknowledgment to the mobile station. The primary base station may intentionally delay the transmission of this access acknowledgment such that the mobile station increases its transmit power level. The primary base station then sends a channel assignment message to the mobile station. The mobile station responds to receipt of the channel assignment message by transmitting a location signal at the increased transmit power level. The location signal may be a traffic preamble corresponding to a known set of data spread by a pseudorandom noise (PN) sequence specific to the mobile station. The primary base station and surrounding base stations detect the traffic preamble and utilize it to generate path delay measurements indicative of the path delay between the mobile station and the base stations. The primary base station intentionally delays the transmission of an acknowledgment of the traffic preamble, such that the mobile station continues to transmit the preamble for a longer period of time than it would otherwise. This extended transmission time allows the base stations to utilize a longer integration period in the detection process, resulting in more accurate path delay measurements. The primary location processor associated with the primary base station receives the aggregate path delay information from the base stations involved in the position estimation, and uses the path delay information to estimate the location of the mobile station. After the base stations have detected the traffic preamble, the primary base station sends an acknowledgment to the mobile station indicating that the channel set up is complete, and the call or page response can then take place over the traffic channel.
In accordance with another aspect of the invention, mobile estimation techniques suitable for use in on-going calls are provided. The mobile estimation techniques may be triggered by a request for mobile location service being received in a primary base station which is processing an on-going call for a particular mobile station. The mobile location service request may be in the form of a signal generated at the expiration of a periodic timer, or a command received from a location service agent. A primary location processor associated with the primary base station then directs a number of surrounding base stations to prepare for detection of a location signal to be transmitted by the mobile station. The location signal may be in the form of a traffic preamble typically generated by the mobile station during set up of the traffic channel for a conventional call.
A first technique suitable for use during on-going calls involves the primary base station transmitting a fake handoff message to the mobile user directing the mobile user to handoff to the same base station and traffic channel, but at a different power level and with a different preamble length. This fake handoff message may be provided using, for example, the extended handoff direction message (EHDM) of an IS-95 CDMA system. Upon receiving the fake handoff message, the mobile station transmits the traffic preamble at the power level and preamble length specified in the message. The preamble is detected in the primary and surrounding base stations to generate the path delay information in the manner previously described. The primary base station then sends a power down command directing the mobile station to reduce its transmit power in order to limit system interference, followed by an acknowledgment of the traffic preamble to complete the fake handoff process. The delay information generated by the various base stations is supplied to the primary location processor associated with the primary base station, and used to estimate the mobile station location. The mobile station stops transmitting the traffic preamble after receipt of the acknowledgment from the primary base station, and the on-going call can then continue over the traffic channel.
A second technique suitable for use during on-going calls involves the primary base station transmitting a location probe message (LPM) to the mobile user directing the mobile user to transmit a traffic preamble or other predetermined location signal at a desired power level and preamble length. Upon receiving the LPM, the mobile station transmits the traffic preamble at the specified power level and preamble length. The mobile station then immediately returns to a conversation mode and reduces its transmit power level to a power level used before sending out the traffic preamble. As in the previously-described technique, the traffic preamble is detected in the primary and surrounding base stations to generate the path delay information. The delay information generated by the various base stations is supplied to the primary location processor associated with the primary base station, and used to estimate the mobile station location. The on-going call continues in the traffic channel after the mobile station completes its transmission of the traffic preamble.
These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.